In the United States alone, there could be up to a million infections a day this winter, Chris Murray, head of the Institute of Health Metrics and Evaluation (IHME), an independent modelling group at the University of Washington that has been tracking the pandemic, told Reuters.

That would be around double the current daily tally.

Across the United Kingdom and Europe, scientists predict a series of COVID-19 waves, as people spend more time indoors during the colder months - this time with nearly no masking or social distancing restrictions in place.

However, while cases may surge again, deaths and hospitalisations are unlikely to rise with the same intensity, the experts said, helped by vaccination and booster drives, previous infection, milder variants and the availability of highly effective treatments.

"The people who are at greatest risk are those who have never seen the virus, and there's almost nobody left," Murray said.

These forecasts raise new questions about when countries will move out of the COVID-19 emergency phase and into a state of endemic disease, where communities with high vaccination rates see smaller outbreaks, possibly on a seasonal basis.

Many experts had predicted that transition would begin in early 2022, but the arrival of the highly mutated Omicron variant disrupted those expectations.

"We need to set aside the idea of 'is the pandemic over?'," Adam Kucharski, an epidemiologist at the London School of Hygiene and Tropical Medicine said.

He and others see COVID-19 morphing into an endemic threat that still causes a high burden of disease.

"Someone once told me the definition of endemicity is that life just gets a bit worse," he said.

The potential wild card remains whether a new variant will emerge that out-competes the currently dominant Omicron sub-variants.

If that variant also causes more severe disease and is better able to evade prior immunity, that would be the "worst-case scenario", according to a recent World Health Organisation Europe report.

"All scenarios (with new variants) indicate the potential for a large future wave at a level that is as bad or worse than the 2020/2021 epidemic waves," said the report, based on a model from Imperial College of London.

Many of the disease experts said making forecasts for COVID-19 has become much harder, as many people rely on rapid at-home tests that are not reported to government health officials, obscuring infection rates.

BA.5, the Omicron sub-variant that is currently causing infections to peak in many regions, is extremely transmissible, meaning many patients hospitalised for other illnesses may test positive for it and be counted among severe cases, even if COVID-19 is not the source of their distress.

Scientists said other unknowns complicating their forecasts include whether a combination of vaccination and COVID-19 infection - so-called hybrid immunity - is providing greater protection for people, as well as how effective booster campaigns may be.

"Anyone who says they can predict the future of this pandemic is either overconfident or lying," David Dowdy, an infectious disease epidemiologist at Johns Hopkins Bloomberg School of Public Health, said.

Experts are closely watching developments in Australia, where a resurgent flu season combined with COVID-19 has been stretching hospitals to breaking point.

They say it is possible other nations could see a similar pattern after several quiet flu seasons.

"If it happens there, it can happen here. Let's prepare for a proper flu season," John McCauley, director of the Worldwide Influenza Centre at the Francis Crick Institute in London, said.

The WHO has said each country still needs to approach new waves with all the tools in the pandemic armory - from vaccinations to interventions, such as testing and social distancing or masking.

Australian Associated Press